Acid is a status effects which inflicts acid on the affected character until the duration of the effects wears or if the character dies from its effects. The amount of damage and duration depends on the means by which it is inflicted. This status effect can be inflicted by the spell Melf's Acid Arrow.
The means to prevent any character from this effect are magic resistance and the spell spell immunity. The items which can prevent this are potions of magic blocking and the scrolls protection from magic and acid.